1981 cb750c DOHC - CYL 2-3 running cold. Help!

[Bob_]

Hi, I have an 81 cb750c with 3,800 miles that I bought as a winter project but I cant quite get it running right. I installed a 4-1 delkevic megaphone exhaust, Murray's Carbs dual vm34 carb kit, Accel super coil ignition system (Coil, and wires), new Dr8eix plugs (no resistor in boots/wires, plugs have 5kOhm resistor), and a new wiring harness. The cyl 2-3 running cold issue however still persist.

Backtracking a little bit, when I installed the new wiring harness, all 4 cylinders started firing correctly. The next day though, I was about to go and fine tune the carbs because I though it was running rich, (Fouled plugs, soot in exhaust, lugging with idle at 1200, only idled smooth at 1500+, and revs hanged if slowly brought up to 5-7k rpm). However, before I could even begin tuning I noticed that 2-3 were once again not firing. (The fouled plugs were cleaned prior).

At this point, I'm kind of at a loss as to what it could be. For further troubleshooting I have switched the spark units with no change, checked all connections, and got 150psi on all cylinders with a compression test, and gapped the pulse generator to 0.013. Timing also appears correct by eye and as stated, 1-4 fire fine. The only 2 things left that I can think of are the pulse generator (I hope not because they don't exist anymore as far as I can see) or the valve clearance, but that seems unlikely due to the compression.

Any advice would be greatly appreciated as this issue is driving me mad.

 

[Rampton]

Sounds like faulty coil??? Worth testing, wishing you good luck

 

[KevinGlover]

Hi, I have an 81 cb750c with 3,800 miles that I bought as a winter project but I cant quite get it running right. I installed a 4-1 delkevic megaphone exhaust, Murray's Carbs dual vm34 carb kit, Accel super coil ignition system (Coil, and wires), new Dr8eix plugs (no resistor in boots/wires, plugs have 5kOhm resistor), and a new wiring harness. The cyl 2-3 running cold issue however still persist.

Backtracking a little bit, when I installed the new wiring harness, all 4 cylinders started firing correctly. The next day though, I was about to go and fine tune the carbs because I though it was running rich, (Fouled plugs, soot in exhaust, lugging with idle at 1200, only idled smooth at 1500+, and revs hanged if slowly brought up to 5-7k rpm). However, before I could even begin tuning I noticed that 2-3 were once again not firing. (The fouled plugs were cleaned prior).

At this point, I'm kind of at a loss as to what it could be. For further troubleshooting I have switched the spark units with no change, checked all connections, and got 150psi on all cylinders with a compression test, and gapped the pulse generator to 0.013. Timing also appears correct by eye and as stated, 1-4 fire fine. The only 2 things left that I can think of are the pulse generator (I hope not because they don't exist anymore as far as I can see) or the valve clearance, but that seems unlikely due to the compression.

Any advice would be greatly appreciated as this issue is driving me mad.

Bob I'll be curious to hear what you find out. I'm having a similar issue and have gone through a similar work-up, although my bike was running well prior to this happening. I was unaware of the pulse generator possibility, I'll have to look into that more.

I also have an 81 CB750C. sync'd carbs, set points and adjusted valve clearance. New plugs. Checked the coils. Good compression in all 4. Still getting cold pipes in 2 and 3.

 

[Bob_]

Bob I'll be curious to hear what you find out. I'm having a similar issue and have gone through a similar work-up, although my bike was running well prior to this happening. I was unaware of the pulse generator possibility, I'll have to look into that more.

I also have an 81 CB750C. sync'd carbs, set points and adjusted valve clearance. New plugs. Checked the coils. Good compression in all 4. Still getting cold pipes in 2 and 3.

I think my issue is going to come down to the carbs running too rich. I got a spark tester and all 4 were able to bridge 20,000v gap on the test so i think that rules out my weak spark/ pulse generator theory. Next week im getting a new valve cover gasket so ill check my valves then too just to get that in order, and then use a timing light to double check timing. After that Ima get new plugs and lean out the air screw as much as recommended and see where that gets me. If that doesnt work I may just admit defeat at get the oem carbs rebuilt.

I made a post on another forum, and amc, who has made a lot of knowledgeable post here basically said the 2 carb setup just aint it for the inline-4's because of how the intakes and firing order are. He also said that they can rob the bikes of around 10hp.

As for yours, are your plugs also coming out fouled as hell? and did you try swapping the spark units? just unplug them and switch em around.

 

[KevinGlover]

I think my issue is going to come down to the carbs running too rich. I got a spark tester and all 4 were able to bridge 20,000v gap on the test so i think that rules out my weak spark/ pulse generator theory. Next week im getting a new valve cover gasket so ill check my valves then too just to get that in order, and then use a timing light to double check timing. After that Ima get new plugs and lean out the air screw as much as recommended and see where that gets me. If that doesnt work I may just admit defeat at get the oem carbs rebuilt.

I made a post on another forum, and amc, who has made a lot of knowledgeable post here basically said the 2 carb setup just aint it for the inline-4's because of how the intakes and firing order are. He also said that they can rob the bikes of around 10hp.

As for yours, are your plugs also coming out fouled as hell? and did you try swapping the spark units? just unplug them and switch em around.

Hey thanks Bob, those are good ideas. I have a 4 carb setup on mine. Plus 1 and 4 are coming out alright, 2 and 3 are coming out dark and wet. When you say swapping the spark units do you mean the coils? I'm not sure if that's different name for the same thing, but yeah, I tried swapping the coils.

 

[Bob_]

Hey thanks Bob, those are good ideas. I have a 4 carb setup on mine. Plus 1 and 4 are coming out alright, 2 and 3 are coming out dark and wet. When you say swapping the spark units do you mean the coils? I'm not sure if that's different name for the same thing, but yeah, I tried swapping the coils.

No, under the seat there are two "spark units" , The oem label on them are "Oki mps-200". Not entirely familiar with the whole system but the most basic explanation is that the pulse generator sends a signal to the spark units and the spark units send a signal to the coil. There are 2 of them under the seat, one for the left, and one for the right coil. You can figure out which one goes where by looking at the wires that feed into their plugs. Blue wire is 1-4 coil, yellow wire is 2-3 coil.
So, simply unplug the 2 units, and switch the plugs around. Plug the unit thats going to 1-4 coil into the 2-3 coil harness and vice versa. If the spark units are the issue then then your issue should move to cyl 2-3.

If you have a manual with electrical diagrams, you can see that the circuit is actually pretty simple.

Also, each unit is identical and should only have one plug coming out of it so there is no harm in swapping them in the same way you swap the coils to test them.

 

[KevinGlover]

Bob, that's really helpful, thank you. I'll look into that! Would be a nice explanation for why 2 and 3 are gone together. I'm reading my Clymer manual right now. You came here for help but instead you're just educating me! Thanks for the help.

No, under the seat there are two "spark units" , The oem label on them are "Oki mps-200". Not entirely familiar with the whole system but the most basic explanation is that the pulse generator sends a signal to the spark units and the spark units send a signal to the coil. There are 2 of them under the seat, one for the left, and one for the right coil. You can figure out which one goes where by looking at the wires that feed into their plugs. Blue wire is 1-4 coil, yellow wire is 2-3 coil.
So, simply unplug the 2 units, and switch the plugs around. Plug the unit thats going to 1-4 coil into the 2-3 coil harness and vice versa. If the spark units are the issue then then your issue should move to cyl 2-3.

If you have a manual with electrical diagrams, you can see that the circuit is actually pretty simple.

Also, each unit is identical and should only have one plug coming out of it so there is no harm in swapping them in the same way you swap the coils to test them.

 

[Shon Ton]

Sounds like faulty coil??? Worth testing, wishing you good luck

Agree I second that and would say that is the coil to cylinders 2 and 3.
Good diagnosis!